The invention disclosed herein relates generally to hermetic electrically insulating seals, and more specifically to a method for producing a hermetic glass to metal compression seal using a dehydrated pure hydrogen environment to avoid metal oxidation.
Hermetic seals between elements are widely required or advantageous in a variety of apparatus, particularly including electrical devices. For example, in electrical switches for use in harsh environments, the switching elements are located in a sealed chamber and hermetic seals are required where feedthrough conductors or terminals pass through the chamber wall. Hermetically sealed switches are frequently provided with metal cases from which the feedthrough conductors must be electrically insulated. It is known to fabricate such switches by utilizing a metal header having apertures therethrough for accommodating the feedthrough conductors and forming the seals around the feedthrough conductors from glass.
A variety of factors must be considered in forming a glass to metal seal for an electrical feedthrough conductor. These include the electrical properties of the material from which the conductor is formed, the thermal coefficients of expansion of the glass and metal parts, the melting or softening temperatures of the glass and metal materials, the suitability of the metals from which the feedthrough conductors and other metal parts are made for subsequent processing steps, such as soldering, brazing, welding, crimping, etc., and the number and complexity of preparatory processes required to permit subsequent fabrication and utilization.
Two general types of hermetic glass to metal seals are known. These are bonded seals in which the molten glass wets and adheres to the metal surface as the glass solidifies, and compression seals in which sealing is accomplished by large compression forces on an inner member, such as a glass bead, by an outer member. Hermetic seals may also employ a combination of these characteristics.
One of the problems encountered with bonded seals where the glass and metal have different thermal coefficients of expansion, as is true for most types of glass and metal, arises from stress concentrations set up in the glass. Since, in a bonded seal, the molten glass wets the surfaces of the contiguous metal parts, the glass forms a concave meniscus leaving a thin glass edge. As the assembly cools and the glass solidifies, stresses are created in the glass due to the different thermal coefficients of expansion. The thin glass edges may not be able to withstand these stresses. The result may be cracks which propagate through the glass elements and jeopardize the hermetic seal.
A concurrent manufacturing disadvantage may accompany the use of bonded hermetic glass to metal seal assemblies in that wetting of the metal surfaces by the molten glass occurs only if the metal surfaces have an oxide formed thereon which occurs normally when the metals from which the metal parts are commonly made are subjected to the melting temperature of the glass in anything but a highly reducing environment. This oxide must be removed for subsequent manufacturing steps, such as brazing or welding housing parts together, making either internal or external soldered, brazed or welded connections to the feedthrough conductors, etc. Removal of the oxidation requires disadvantageous cleaning steps in the manufacturing process.
The applicant has discovered that a satisfactory hermetic glass to metal seal can be achieved by selecting glass and metal materials to result in a compression seal, and conducting the processes to form the seal in a highly reducing environment which avoids the formation of thin glass sections subject to stress cracking and the formation of oxide whose removal requires disadvantageous manufacturing steps.